Neutrino fast flavor oscillations with moments: linear stability analysis and application to neutron star mergers

Providing an accurate modeling of neutrino physics in dense astrophysical environments such as binary neutron star mergers presents a challenge for hydrodynamic simulations. Nevertheless, understanding how flavor transformation can occur and affect the dynamics, the mass ejection, and the nucleosynthesis will need to be achieved in the future. Computationally expensive, large-scale simulations frequently evolve the first classical angular moments of the neutrino distributions. By promoting these quantities to matrices in flavor space, we introduce a linear stability analysis of fast flavor oscillations using only the first two “quantum” moments (i.e., number densities and fluxes). This notably requires generalizing the classical closure relations that appropriately truncate the hierarchy of moment equations in order to treat quantum flavor coherence.

In this talk, I will present the results of a systematic search of the occurrence of fast flavor instabilities in a neutron star merger simulation using our linear stability analysis with moments. This framework, which has some clear successes and shortcomings, provides a time-efficient way to guide future moment calculations and to design and study better closure prescriptions, necessary for accurate predictions of the amount of flavor transformation.